1. Field of the Invention
The invention relates to a system for acquiring a fingerprint image or the like object surface pattern. More specifically, it utilizes a layer of elastic microprisms as a sensing element for reproduction of a distortionless ridge valley pattern and convert the pattern data into a digital form for further storage or analysis.
2. Description of the Prior Art
FIG. 1 illustrates the phenomenon of reflection or refraction of light rays when traveling through two different optical substances. The substance I is a transparent material whose refractive index is Ng; Substance II is another kind of material with a refractive index Ns; The substance III is the air whose refractive index Na is 1. When light ray e strikes the boundary between substances I and II at point d at an incidence angle Og, it is refracted into substance II at an angle Os, denoted as light ray f. Here the refractive indices Ng and Ns and the angles Og and Os must satisfy Snell's Law, i.e., EQU Ng.times.sin Og=Ns.times.sin Os,
where sin denotes the sine function. Similarly, if the refractive index Na of substance III is smaller than Ng.times.sin Og, then light ray b will be totally reflected into substance I, when it strikes the boundary between substances I and III at an incidence angle Og. The reflection angle is also equal to Og. In a fingerprint acquisition application, substance I is a transparent sensing element against which a finger to be imaged is placed; substance II is sweat on the ridge portion of a finger and substance III is the air trapped by the valley portion of a finger. Quite many fingerprint imaging apparatus contain a transparent element in the form of a block (or truncated) prism which has a contact face against which is placed a finger to be imaged. The finger surface is illuminated by light entering from another face of the prism. Apparatus utilizing the principle of the total internal reflection to obtain the ridge and valley pattern of the finger are disclosed in a number of patents including U.S. Pat. Nos. 4,120,585 and 4,905,293. The image has a poor contrast with a glass prism, when the finger skin is dry. If a bigger pressure is exerted, a clearer image is obtainable, but with a severe distortion.
In these apparatus the aspect ratio of the ridge/valley image is not 1:1, namely, the horizontal and vertical scales in the image are not equal. This is due to the different lengths of optical paths between the points on the finger surface and the non-parallel viewing face of the prism through which a photo-detector or a CCD camera captures the fingerprint image. To facilitate obtaining a clearer fingerprint image without applying a big finger pressure, U.S. Pat. Nos. 5,051,576, 4,428,670 and 5,096,290 disclose the use of a layer of liquid or elastic material provided on the finger contact face of the prism. The refractive indices of the two materials are chosen to be matched as closely as possible. On the other hand, to obtain the 1:1 aspect ratio image of the fingerprint, U.S. Pat. Nos. 4,553,837 and 4,537,484 disclose the use of a stationary cylindrical-segment platen for receiving a finger to be imaged and a stepper motor driven rotatable carriage housing the light source and photo-detector to generate line by line a roll fingerprint image. It generally takes time to obtain the image of a single finger.